WHAT’S THE CONNECTION? Sharon Winter Lake Washington High School Directions for Teachers 12033 NE 80th Street Kirkland, WA 98033 SYNOPSIS Students elicit and observe reflex responses and distinguish between types STUDENT PRIOR KNOWL- of reflexes. They then design and conduct experiments to learn more about EDGE reflexes and their control by the nervous system. Before participating in this LEVEL activity students should be able to: Exploration, Concept/Term Introduction Phases ■ Describe the parts of a Application Phase neuron and explain their functions. ■ Distinguish between sensory and motor neurons. Getting Ready ■ Describe briefly the See sidebars for additional information regarding preparation of this lab. organization of the nervous system. Directions for Setting Up the Lab General: INTEGRATION Into the Biology Curriculum ■ Make an “X” on the chalkboard for the teacher-led introduction. ■ Health ■ Photocopy the Directions for Students pages. ■ Biology I, II ■ Human Anatomy and Teacher Background Physiology A reflex is an involuntary neural response to a specific sensory stimulus ■ AP Biology that threatens the survival or homeostatic state of an organism. Reflexes Across the Curriculum exist in the most primitive of species, usually with a protective function for ■ Mathematics animals when they encounter external and internal stimuli. A primitive ■ Physics ■ example of this protective reflex is the gill withdrawal reflex of the sea slug Psychology Aplysia. In humans and other vertebrates, protective reflexes have been OBJECTIVES maintained and expanded in number. Examples are the gag reflex that At the end of this activity, occurs when objects touch the sides students will be able to: or the back of the throat, and the carotid sinus reflex that restores blood ■ Identify common reflexes pressure to normal when baroreceptors detect an increase in blood pressure. of the human body and the neural circuitry involved in A second type of reflex, the stretch reflex, has evolved to help maintain these reflexes. muscle tone that is important for posture and movement. ■ Distinguish between An understanding of the neural circuitry underlying each type of reflex also stretch and protective has clinical significance. When physicians test for potential damage to reflexes. ■ Describe the role of various components of the nervous system, they often begin by attempting reflexes. to elicit reflex responses to the appropriate stimuli. Examples of reflexes with protective and diagnostic importance are the flexor withdrawal reflex, the corneal (blink) reflex, and the accommodation reflex. The loss of the pupillary light reflex in a comatose patient, described under the section Protective reflex, often indicates extreme damage deep in the brain. Loss of the knee-jerk response may indicate problems with muscle tone, chronic diabetes, or damage at the L2, L3, or L4 vertebral level. What’s the Connection? 59 LENGTH OF LAB A suggested time allotment follows: Day 1 5 minutes — Teacher introduction. 40 minutes — Conduct the activity and assign research. Day 2 40 minutes — Analyze data and develop hypoth- Figure 1. Components of neural circuitry of reflexes. eses. 5 minutes — Select a For each reflex resulting in a specific motor output, the neural pathway hypothesis for experi- linking the sensory receptor to the central nervous system (CNS) may be mental design. studied. In this activity, components of the neural circuitry of reflexes will Day 3 be studied. These components include a sensory receptor, a sensory (afferent) 45 minutes — Design and neuron, a reflex center in the CNS that may involve one or more levels of conduct experiments. integration, a motor (efferent) neuron, and a muscle (effector). See Figure 1. Day 4 Students will investigate the following reflexes to learn about these compo- 45 minutes — Analyze nents: and discuss results of ■ Stretch reflex: the knee-jerk (patellar) reflex. See Figure 2. Tapping experiments. the patellar tendon stretches striated muscle fibers in the quadriceps, resulting in activation of specialized sensory receptors within the MATERIALS NEEDED muscle called muscle spindles. Sensory neurons connected to the You may need one or more of muscle spindles are activated and carry the stimulus to the spinal the following for the teacher- cord, where they synapse onto a motor neuron. These motor neurons led introduction: are triggered by a neurotransmitter and release a neurotransmitter ■ 1 book ■ 1 balloon back at the quadriceps muscle in a synaptic area of the striated ■ 1 pin muscle membrane called the neuromuscular junction. This results in ■ 1 watch/clock with a shortening the length of the muscle fibers and knee extension. The second timer knee-jerk and other tendon reflexes differ significantly from other You will need the follow- types of reflexes in that no central nervous system interneurons are ing for each group of four involved in the knee-jerk reflex. Under normal circumstances, brain students in a class of 24: pathways descend onto the same motor neurons, constantly modulat- ■ 1 rubber percussion ing the strength of stretch reflexes. This modulation of muscle tone hammer or a 30-cm ruler keeps muscles from being damaged during everyday movements. This ■ 1 utility flashlight or process occurs not only in the quadriceps muscle, but also in other penlight areas of the body as well. If students experience mild discomfort while ■ 1 pair safety goggles ■ 1 square of light card- participating in this activity, it is important to emphasize that this board (5 x 5 cm) discomfort was felt only after the reflex response. Pain perception is ■ 1 marker processed by the brain and cannot be experienced until the message has had enough time to travel to the brain. — Continued 60 Neuroscience Laboratory Activities Manual Figure 2. Stretch reflex. ■ Protective reflex: the pupillary light reflex. Shining a light into one eye activates sensory receptors in the retina of that eye, eventually MATERIALS stimulating output cells of the retina called ganglion cells. Refer to — Continued the lab “Is Seeing Believing?” on p. 239 for more information about ■ 1 piece of poster board ganglion cells. The ganglion cells respond to brightness in the eye and (70 x 55 cm) ■ 1 protractor send the signals via the optic nerve to the midbrain, located in the (optional) brainstem. Some optic nerve fibers synapse in the midbrain on the same side that the eye was stimulated, while others cross and synapse PREPARATION TIME in the midbrain on the opposite side. As a result, the neurons in the REQUIRED midbrain process the signal on both sides of the brain. See Figure 3. 30 minutes to locate and Outflow from these neurons exits the brain within both oculomotor obtain materials, if the nerves, but does not go to the relay centers in the thalamus that send hammers and penlights are signals to the cerebral cortex for visual processing. Components of available these nerves devoted to the pupillary light reflex synapse on parasym- pathetic neurons outside of the eye, activating the smooth muscle SAFETY NOTES ■ Supervise students cells of the pupillary sphincter muscle. Pupils of both eyes constrict carefully to ensure that due to the bilateral processing in the midbrain. There is a direct they are tapping their pupillary light reflex in the eye that was originally stimulated and a partners’ knees lightly consensual pupillary light reflex in the other eye. One cannot sup- with the percussion press consciously this protective reflex. See Figure 4. This lab activity hammers. ■ The penlights should be could lead to discussions of the somatic versus autonomic nervous brought only within 5 to 10 systems, control of skeletal versus smooth muscle, and downstream cm of the eye and should modulation/control of neural pathways by the cerebral cortex. not shine on the eye for more than 3 to 5 seconds. ■ Safety goggles should be worn. What’s the Connection? 61 TEACHING TIPS ■ Percussion hammers are available from science education supply catalogs; 30-cm rulers are an acceptable substitute. ■ Many students probably have small flashlights at home that they can bring to school. ■ Some students may need to practice with the hammer a few times to find the appropriate place to tap their partner’s knee in order to elicit the correct knee-jerk action. Some students may experience mild discomfort from the knee tap. SAMPLE HYPOTHESIS Figure 3. Reflex pathway mediating pupillary light reflex. The knee-jerk reflex can be suppressed consciously Procedure by an individual. Some suggestions follow for introducing this activity: SAMPLE PROCEDURE ■ Have students stare at an “X” drawn on the chalkboard for as long as Repeat Steps 1 through 7 of they can before blinking. the Exploration section of ■ Drop a book on the floor. this activity. The only change will be that the subject will try consciously to keep the leg from responding to the light tap of the hammer. SUGGESTED MODIFICATIONS FOR Figure 4. Diagram of bilateral processing in the midbrain. 62 Neuroscience Laboratory Activities Manual ■ Close a book with a loud smack. ■ Pop a balloon that is sitting on the teacher’s demonstration table. STUDENTS WHO ARE ■ EXCEPTIONAL Pretend to throw an imaginary object at the class. Below are possible ways to Whatever stimulus you choose to use, take care not to compromise the safety modify this specific activity for of your students. Immediately following the reflex response, ask questions students who have special suggested in the list below: needs, if they have not already developed their own adapta- ■ What was your reaction to the stimulus? tions. General suggestions for modification of activities for ■ How did your body respond? students with impairments ■ How quickly did it respond? are found in the AAAS Barrier- ■ Can you control your reaction? Free in Brief publications. Refer to p. 19 of the introduc- ■ What is the function of this response? tion of this book for informa- ■ What is this response called? If students use the word “reflex” in tion on ordering FREE copies answering this question, ask them to name other reflexes.